As discussed in U.S. Pat. No. 5,444,776 to Sheets, NIUs are typically located at the interface between a network transmission line and a customer premises. In general operation, when a NIU receives a signal on a digital transmission line from the central office, the NIU, in turn, passes the signal to equipment on the customer premises. Similarly, when the NIU receives a signal from the customer premises, the NIU passes that signal to the central office on the digital transmission line.
The present invention may be used with digital transmission lines generally, including, for example, the Regional Bell Telephone Systems in the United States. The Bell Telephone System has widely utilized time multiplexed pulse code modulation systems. Such systems have generally been designated as "T1 carriers." Each T1 system carries twenty-four two way channels on two pairs of exchange grade cables. One pair of cables provides communication in each direction. The information on such a pulse code modulated system is transmitted in the form of bipolar or alternate mark inversion (AMI) pulses.
The data to be transmitted over the cables, such as speech, is typically sampled at a rate of 8,000 Hertz, and the amplitude of each signal is measured. The amplitude of each sample is compared to a scale of discrete values and assigned a numeric value. Each discrete value is then encoded into a binary form. Representative binary pulses appear on the transmission lines.
Payload signals in binary pulse form are received by the telephone company central office and, generally, are transmitted, via cables, to a series of regenerative signal repeaters ("line repeaters" or "signal repeaters"). Such repeaters are spaced along the cables approximately every mile. The first repeater receives the data from the central office, but, because of transmission line losses, noise, interference, and distortion, the signal will have degraded. The repeater recognizes the presence or absence of a pulse at a particular point in time and, thereafter, if appropriate, regenerates a clean, new pulse. Regenerative repeaters are powered by an office repeater through the transmission cable itself to generate the new pulses. The new pulses are transmitted by the line repeater along more cable to either another line repeater or to a NIU.
Some NIUs have the capability to identify errors in the data received over the cable and responsively provide a signal to the central office that the errors have occurred. Errors that can be detected by the NIU include, for example, errors in signaling, format, bipolar violations, out of frame data, loss in signal or loss of power, as well as the disconnection of equipment by the customer. NIUs may include regeneration toward the customer premises. Similarly, a NIU may include regeneration in the opposite direction.
NIUs may also include a "loopback" feature that allows the central office to ascertain whether or not a particular span of cable provides continuity along its entire length. For example, the central office may send, via the digital transmission lines, an activating signal, which may or may not interfere with normal transmission operations, that designates the NIU to "loop back" a signal to the central office. If no break is present on transmission lines, the central office test equipment will receive the same signal that it transmitted. The central office test equipment can now compare the transmitted signal to the received signal to obtain an accurate error count. Efficient information gathering, system monitoring and status reporting are obviously desirable in an NIU. Furthermore, the ablity to embed this gathered information back into the data stream for collection by a centrally located, up stream device is also desirable. NIUs currently available, however, are prevented from modifying the digital stream primarily due to signal level transmission requirements put in place by the FCC. For competitve reasons, the details of which are not important here, FCC regulations inherently require an NIU to appear transparent to external equipment. In other words, the level of signal going into the NIU must be substantially the same as the level of the corresponding signal which is transmitted by the NIU. Lone Build Out (LBO) circuitry in the customer equipment sets the signal to the proper transmission level before it enters the network boundary at the network interface. This level must be properly set so that the signal closely matches other signals in the network binder cables that carry the DS-1 lines to the first line repeater locations. Matching the signal levels decreases the chance of crosstalk occurring in the binder groups, the signal level must also be adjusted so that the level is within the operating range of the line repeater. Line repeaters typically can only handle -7.5 dB to -35 dB. Line Build Out (LBO) circuitry in customer equipment, such as Customer Service Units (CSU), sets the signal to the proper transmission level before it enters the network boundary at the Network Interface. This level must be properly set so that the signal closely matches other signals in the network binder cables that carry the DS-1 lines to the first Line Repeater location. Matching the signal levels decreases the chance of crosstalk occurring in the binder groups. The signal level must also be adjusted so that the level is within the operating range of the Line Repeater. If a NIU, which must regenerate digital signals at 0 dB reference levels in order to process an incoming T1 data stream, transmits signals at the 0 dB reference level, line repeaters may function improperly and crosstalk may be introduced into the binder cables. Consequently, it has been impossible to effectively process a binary pulse data stream in existing NIUs without substantially affecting dB reference levels, and in turn network equipment.
There are disclosed in the art systems for remotely monitoring and testing the performance of telephone circuits (U.S. Pat. No. 5,566,161 to Hartmann et al.); for maintaining signal levels over remote lines (U.S. Pat. No. 3,496,308 to Godfrey); for regenerating signals without introducing frequency shift or changing signal direction (U.S. Pat. No. 3,863,031 to Cook); for amplifying, re-shaping and retiming signals (U.S. Pat. No. 4,821,286 to Graczyk); and for monitoring signal transmission for purposes of communication and maintenance purposes such as framing detection (U.S. Pat. No. 5,521,977 to Bergstrom).
The Hartmann '161 patent discloses a network interface unit that regenerates and processes incoming digital signals to monitor and test the performance of telephone circuits.
The Godfrey '308 patent discloses a system for maintaining signal levels of telephone messages over remote lines. The gain of amplifiers placed at repeaters and remote stations are automatically adjusted to compensate for variations and losses in transmission lines. The system compensates automatically for all fluctuation in the lines which causes a need for gain control. To accomplish this, the signal coming from a central office is monitored continuously, with the control device increasing or decreasing the gain of repeaters as necessary.
The Cook '031 patent discloses a signal processing system whereby a pulse or pulses of predetermined frequency and duration are filtered and regenerated, with the regenerated signal closely approximating the frequency and duration of the original pulse.
The Graczyk '286 patent is a digital signal regenerator which receives a four-level input signal and amplifies, reshapes and retimes that signal into a regenerated four-level output signal. The four-level signal system ostensibly increases the rate of information flow without substantially increasing the operational frequency.
The Bergstrom '977 patent discloses a network interface unit requiring less shelf space than the existing network interface units. Utilizing a single planar circuit board assembly to connect incoming and outgoing telephone lines, a higher density network interface unit is achieved to provide and monitor framing, detecting and monitoring signals and converting bipolar data to unipolar data.
None of the above-described systems discloses a NIU which accepts one or more high-speed digital signals, inputs information into the signals and dynamically regenerates the signals to their original wave shape and amplification, such that the processing of the NIU is transparent to external equipment. A system encompassing such features would result in a more "intelligent" NIU, allowing more efficient transmission line monitoring and the ability to communicate with other network elements at more points along the transmission system. Accordingly, there is a need in the art for a NIU which processes and inputs data into a digital data stream for improved monitoring and detection purposes, and regenerates the original signal such that the function of the NIU is transparent to external equipment.